Wider tires perform worse in snow since the larger surface area of the tire reduces its ability to cut in and create traction. In most cases when you get winter tires they don't come in 8" widths, 7" is the largest I've seen. His DSC is flashing because his BMW is RWD and he has wider tires there.

It's always possible to put a smaller rim diameter on your car, you just need tires with thicker walls to accommodate. If your 17" rims are 5 lugnut designed and not some crazy offset I bet they will work fine.

so the front wheel of the M3 does not have the same size of the rear wheel.

In the set used by the M3 owner in the post above, he has choosen the same wheels for front and rear wheel -> this is why the DSC is flashing all the time, as it has been configured for 2 different size of wheels.

Thooooorgal, are you buying your winter tyres straight from BMW or somewhere else in Luxembourg (Goedert for ex) for your M3 ? As far as I know, the BMW "winter kit" for M3 is not yet available and we have exactly the same problem, our M3 should arrive on the 3rd week of January and I don't really want to drive it without winter tyres, specially if there's snow/ice.

Indeed . We'll keep you posted about the tyres. Our BMW dealer at Esch sur Alzette gave us an estimation for the tyres + rims (around 1800 €), but I'd really like to see the rims, because we wouldn't want anything that doesn't match the glorious all black M3 now, would we ?

Thoooooooorgal, this is what I received this morning from BMW:
"L'action pneu hiver est sur du Michelin et les jantes sont les jantes d'origine, comme sur votre commande. Le prix est de 2400€ttc". I have asked them to confirm me the exact type of Michelin tyres as well as the exact size. Then we can go hunting somewhere else...

Thoooooooorgal, this is what I received this morning from BMW:
"L'action pneu hiver est sur du Michelin et les jantes sont les jantes d'origine, comme sur votre commande. Le prix est de 2400€ttc". I have asked them to confirm me the exact type of Michelin tyres as well as the exact size. Then we can go hunting somewhere else...